The goal of this Program Project is to elucidate the hormonal, molecular, and cellular mechanisms involved in two critical developmental transitions in the mammary gland: those from pregnancy to lactation, and from lactation to the return of the lactating gland to its pre-pregnant state at involution. The proposed research is designed to provide an integrated and cooperative approach to these problems. In Project I, the relationship between progesterone withdrawal and tight junction closure during the transition between pregnancy and lactation will be examined using state of the art genomic and proteomic technology. In Project II the detailed molecular mechanisms by which progesterone inhibits milk secretion during pregnancy will be elucidated focusing on interactions between the progesterone receptor, the glucocorticoid receptor and the JAK/STAT signal transduction pathway. In Project III the molecules that regulate the physical expansion of the Golgi and the onset of mil lipid secretion during the transition to lactation will be identified and their functional significance assessed. Finally, in Project IV the mechanisms by which mammary epithelial cells and macrophages remove apoptotic cells from the involuting mammary gland in the absence of inflammation will be studied. Three cores will provide (A) administrative support, (B) specialized animal breeding and surgical support and (C) adenovirus vectors for use in transduction of genes designed to alter functional pathways. Experiments will be carried out in vitro and in vivo using advanced techniques to examine changes in gene expression and protein composition during these well-define developmental transitions of the epithelium from one physiological state to another. These studies should enhance our understanding of developmental and secretory regulation in the mammary gland and provide an excellent model for the regulation of similar processes in many organs. Because milk is critically important to infant nutrition, better insight into these transitions should lead to improve infant nutrition as well as contribute to the prevention and cure of breast diseases.